Gear differentials include compound planetary gear sets carried within a differential housing for interconnecting a pair of output drive shafts in a manner that permits the two drive shafts to rotate in opposite directions with respect to the housing. Sun gear members of the respective planetary gear sets, also referred to as "side" or "end" gears, are coupled to inner ends of the two drive shafts. Planet gear members of the respective planetary gear sets, also referred to as "element" or "spider" gears, interconnect the two sun gears for rotation in opposite directions.
The sun gears are coupled to the respective drive shafts for rotation about a common axis. However, the planet gears are mounted for rotation about axes that can be variously offset and inclined with respect to the common axis of the sun gears and drive shafts. In fact, the relative positions of the sun and planet gear axes in large part determine the kind of gearing that make up the planetary sets. For example, spur or helical gears are used when the sun and plant axes are parallel. However, when the axes intersect at right angles, bevel gears are used. Worm or helical gears are used when the axes are inclined but do not intersect.
Some gear differentials that are made with worm or helical gears include planet members that are formed as "combination" gears having at least two gear portions. One portion of each combination gear meshes with one of the sun gears, and the other portion of each combination gear meshes with another combination gear. The combination gears are mounted in pairs about the periphery of the sun gears. Each pair of combination gears forms a separate gear train interconnecting the sun gears. The separate gear trains increase power transmitting capabilities of gear differentials but also complicate design and assembly of the planetary gear sets.
Once one of the pairs of combination gears has been assembled for interconnecting the sun gears, relative rotational positions between the sun gears are established; and the other pairs of combination gears must be assembled in a manner that preserves this established relationship between sun gears. Otherwise, the other pairs of combination gears will not fit properly into mesh. Moreover, not all combinations of gear tooth numbers for a given number of combination gear pairs will enable all of the combination gears to fit properly into mesh. In other words, the gear trains must be designed with regard to the tooth numbers of their gear members to enable the gear trains to fit into mesh at particular rotational positions between the members. Relationships between the combination gear pairs of different gear trains required to fit the combination gears properly into mesh are referred to as "timing".
In worm gear differentials of the type disclosed in US-A-2 859 641 (GLEASMAN), timing requirements are met by mounting each combination gear in a particular order and at a distinct rotational position with respect to other combination gears. Such particular mounting positions cannot be readily ascertained from the mere appearance of the combination gears, and a large number of possible mounting positions that do not meet the timing requirements can make trial and error assembly procedures impractical. There is also a danger that combination gears could be fit into mesh at inexact positions that would prevent the gear trains from sharing equal loads.
Accordingly, special assembly procedures have been used to ensure that the gear trains are properly assembled. Examples of such procedures are found in a series of patents commonly owned herewith identified as US-A-3 849 862, US-A-3 875 824, and US-A-3 902 237 (all to BENJAMIN). These patents disclose use of reference marks that are placed in predetermined positions on the combination gears. The reference marks are used to help index each combination gear with respect to the other combination gears by predetermined angular amounts that allow all of the gears to be fit into mesh.
These assembly procedures using reference marks are cumbersome and time consuming. Typically, it is necessary to identify the reference mark on each combination gear and to mount the gear in a particular rotational position following a prescribed sequence with the other combination gears. The particular rotational positions at which the combination gears are assembled can vary significantly between different arrangements of tooth numbers in the combination gears.
The combination gears of the known worm gear differentials have respective middle gear portions for meshing with one of two side gears and two end portions for meshing with a paired combination gear. The two end portions have respective gear teeth that are indexed relative to each other, usually by one-quarter circular pitch. However, combination gear members of each pair have respective end portions that are indexed in opposite directions for fitting the two end portions of the combination gear members of each pair into mesh. The different combination gear members of each pair further complicate assembly of the known worm gear differentials by requiring the combination gears to be sorted prior to mounting them in a particular rotational position.
However, a commonly owned copending U.S. patent application Ser. No. 460,131, now U.S. Pat. No. 5,088,970, filed on May 25, 1990, entitled Timing of Multiple Gear Train Differential, proposes rules to fit combination gear pairs properly into mesh without using special timing marks on the gears. Other rules enable both members of the combination gear pairs to share a common gear design and to be mounted in either of two axial orientations within the differential. This copending application is hereby incorporated by reference.
According to one of the rules proposed in the copending application, the number of teeth in each end gear portion must be an integer multiple of the number of teeth in the middle gear portion. Another rule requires that twice the product of this integer multiple and the number of side gear teeth all divided by the number of combination gear pairs must equal an integer. The latter rule assures that it is possible to assemble the separate gear trains in properly timed positions. The former rule in concert with the latter rule assures that it is possible to fit any one of the middle gear teeth into mesh with teeth in the side gears. Although not all of the end gear teeth will fit into mesh, the two rules also assure that any end gear teeth that do fit into mesh are properly timed with respect to all other of the combination gears. The end gear teeth that do not fit into mesh are associated with rotational errors between the teeth of the middle gear portions and the side gear teeth at least equal to the angular spacing between the end gear teeth. This prevents improper assembly of the combination gears.
Although the two rules proposed in the copending application provide for eliminating special reference marks and for preventing misassembly of combination gears in improperly timed positions, the rules severely limit the selection of tooth numbers for the side and combination gears. For example, the requirement for an integer multiple relationship between middle and end gear teeth of the combination gears limits the numbers of end gear teeth that are usable with any one number of middle gear teeth. Under most practical circumstances, the choice of end gear teeth is further limited to multiples of two or three times the number of middle gear teeth. However, different end gear tooth numbers are often preferred because of strength considerations or size constraints.
The copending application proposes another rule that requires the teeth at opposite ends of the combination gears to be indexed with respect to each other through one-half circular pitch. This assures that the combination gear members of each pair have the same design and thus are interchangeable. Yet another rule requires all of the middle gear teeth to be located in positions that bisect the one-half pitch index between the end gear teeth. This assures that the combination gears can be mounted in either of two axial orientations within the differential and thus are invertible.
The two rules that enable the combination gears to be both interchangeable and invertible depend from the two earlier mentioned rules that limit the tooth number combinations of the side and combination gears. For example, it is not possible for all of the middle gear teeth to bisect the one-half circular pitch index between opposite end gear teeth unless the number of gear teeth at each end is limited to an integer multiple of the number of middle gear teeth. Further, there is no advantage to making the combination gears interchangeable or invertible unless the combination gears can be assembled together in a properly timed relationship for interconnecting the side gears.